This invention relates to a method of forming high strength aluminum alloys, particularly in sheet form. The term "aluminum alloy(s)" as used herein refers to those aluminum alloys which have a "yield strength" (YS), after forming, of at least 25,000 psi. That is, the YS of alloys treated according to this invention may increase or decrease as a result of treatment but the final value will always be in excess of about 25,000 p.s.i. For the sake of convenience in laboratory testing, "hardness" (R.sub.B) of the alloys tested and described herein was primarily used as a measuring parameter for test purposes. R.sub.B directly relates to YS and does not require the complicated procedures and handling necessary for making measurements as do YS determinations. The term "sheet" as used herein refers to a flat rolled product in coils or cut lengths having a thickness range of about 0.010 inches to about 0.250 inches in thickness. The term "high strength" in the sense used herein refers to aluminum alloys which obtain their high strength properties by heat treatment and/or by being cold worked into a tempered condition.
High strength aluminum alloys, particularly sheet, are desirable for use in automobiles manufactured because of their light weight and potential decorative appearance. For example, high strength aluminum alloy sheet provides decorative trim moldings which are more durable, have more resistance to denting, and are more resilient for snap-on attachment than comparable parts made of lower strength aluminum or aluminum alloys. However, high strength aluminum alloy sheet has the potential of being primarily useful for structural parts, such as bumpers, bumper reinforcements, body panels, brackets and the like.
Forming high strength aluminum alloy sheet by standard press forming at ambient temperatures, as has been the standard practice to-date, is difficult because of the relatively low ductility of these alloys. During the forming operation the alloys tend to tear and/or fracture.
One approach which has been used for obtaining high strength aluminum alloy parts has been to start with an annealed low strength aluminum alloy, cold form it to a desired shape and then heat treat it to provide a part having high strength. However, the heat treatment then adds complexities to the manufacturing process and increases the piece cost of such parts. Also, heat treating to achieve high strength requires the use of energy, electrical or gas, and it is desirable to minimize energy use where possible.
The object of this invention is to provide improved methods for forming high strength aluminum alloys, particularly in sheet form.